Notably, 65-year-olds in "rural areas have had much smaller improvements than those in large metro areas," Preston remarked. "And people living in 'interior' regions ― particularly Appalachia and the East South Central region [Alabama, Kentucky, Mississippi and Tennessee] — have done worse than those on the coasts."

These geographic differences emerged around 1999-2000 and widened from 2000 to 2016, the study found. By the end of this period, life expectancy at age 65 for women in large metropolitan areas was 1.63 years longer than for those in rural areas. For men, the gap was 1.42 years.

Differences were even starker when 65-year-olds who live in metro areas in the Pacific region (the group with the best results) were compared with their rural counterparts in the East South Central region (the group with the worst results). By 2016, seniors in the first group lived almost four years longer. (The Pacific region includes Alaska, California, Hawaii, Oregon and Washington.)

"Areas with the highest life expectancies at age 65 have realized more significant improvements between 2000 and 2016, while areas with the lowest life expectancies have gained the least," said Yana Vierboom, a co-author of the new study and a postdoctoral researcher at the Max Planck Institute for Demographic Research in Germany.

Disparities were also highlighted when researchers examined life expectancy at 65 in the U.S. and 16 other developed nations, using 2016 data. Overall, the U.S. was near the bottom of the pack: American men ranked 11th while American women were in 13th place, behind leaders such as Japan, Switzerland, Australia, France, Spain and Canada.

But when only 65-year-old American men living in Pacific region metro areas were considered, they topped all other countries, with an added life expectancy of 20.03 years. Women from this advantaged group also jumped in the rankings to the No. 4 position, with a life expectancy of 22.79 additional years.

Pockets of this country "have a life expectancy at 65, which is on par" with that of leading countries, Jennifer Karas Montez, a professor of sociology at Syracuse University, wrote in an email. "We need to figure out what those places are doing right and then take those lessons and apply them to other parts of the country that are doing poorly."

What distinguishes areas that are doing well from those that aren't?

According to the new study, the most important factor is a reduction in deaths from cardiovascular illnesses, such as heart attacks and strokes — the biggest killer in America.

"It's likely that medical treatments for cardiovascular disease have disseminated more rapidly in large metro areas than in rural areas," which have fewer specialist physicians and hospitals, Preston said.

The second-most important factor, especially for women, was smoking, a contributor to cardiovascular disease, lung cancer and respiratory diseases.

"There are large differences in smoking rates across the country," with more women in the South and rural areas taking up smoking and more women in metro areas who've given up the habit, Vierboom said.

While the analysis that Preston and Vierboom conducted didn't examine race, income or education, it's certain that these factors play a part in its findings.

"Geographic differentiation isn't random: People who are poor, or who smoke or who are obese tend to be concentrated in certain places," said Eileen Crimmins, AARP professor of gerontology at the University of Southern California.

Meanwhile, the culture of different areas ― what people see others around them doing, the habits they adopt — tends to perpetuate these differences over time.

While enormous attention has been paid to "deaths of despair" in the younger and middle-aged population, the "real action" regarding mortality is with the 65-and-older population, Crimmins said. Of nearly 3 million people who die each year in the U.S., almost three-quarters are age 65 or older.

Deaths from opioids, alcohol or suicide aren't significant in the older population; instead, deaths from chronic illnesses, which take years to develop and which are influenced by social conditions as well as personal behaviors, are far more important, Preston noted.

This helps explain another notable trend spotlighted in his new research: Life expectancy at age 65 has steadily increased, even in an era when "deaths of despair" have been on the rise.

The long-term trend is upward. In 1950, a 65-year-old could expect to live an additional 13.9 years, on average (15 more years for women, 12 for men). A half-century later, in 2000, life expectancy at age 65 had climbed to 17.6 additional years (19 for women, 16 for men). By 2018, it increased again, adding 19.5 years (20.7 for women, 18.1 for men).

This positive trend has persisted even as death rates due to drug and alcohol abuse, suicide and chronic conditions, such as hypertension and diabetes, rose for middle-aged adults over the past decade. With this surge in midlife deaths, overall life expectancy (starting at birth) in the U.S. declined from 2014 to 2017, followed by a slight uptick in 2018.

"I'm struck by how well older adults are doing because it contrasts with what's happening at midlife," said Anna Zajacova, an associate professor of sociology and a population health expert at the University of Western Ontario.

Why have older adults seen consistent life expectancy gains?

Much credit undoubtedly goes to medical advances and to Medicare, which extended health insurance coverage to all older Americans (age 65 and up) in 1966, improving access to care, said Scott Lynch, a sociology professor at Duke University and training director of Duke's Population Research Institute. By contrast, tens of millions of younger and middle-aged adults are uninsured or underinsured.

Also, Social Security probably makes a difference by providing a minimum income — albeit one that hasn't kept up with rising costs ― for most older Americans.

"Thank the Lord for social insurance programs above age 65," David Cutler, a professor of applied economics at Harvard University, wrote in an email, while acknowledging that experts haven't yet come up with definitive explanations for mortality trends in the older population.

But whether life expectancy trends at age 65 will remain on an upward trajectory is an open question.

In particular, "it's yet to be determined what impact the explosion of obesity among prime-age adults will have when this population passes age 65," said Timothy Waidmann, an economist and senior fellow at the Urban Institute. "My guess is it won't be good. But that's a story yet to be seen."

Kaiser Health News is a nonprofit news service covering health issues. It is an editorially independent program of the Kaiser Family Foundation, which is not affiliated with Kaiser Permanente.

Read more: 

• 5 groups that put retirement at risk by incorrectly estimating their life expectancy
• Rising life expectancy boosts late-life risks for retirees
• Top 5, bottom 5 countries for life expectancy and where the U.S. ranks